text:
Why are planes generally painted white?
When planes come off the assembly line at the factory, they're green:

So why are most painted white? Surely a darker colour would hide dirt better?

Is there a reason planes are traditionally white?

Here's a clip that shows a Boeing 747 being painted:

https://www.google.com/search?q=colorful+planes&tbm=isch - plenty of planes are not white at all.

@oakad This is the reason for the word "generally" in the title :)

It's not a primary reason, but hiding dirt is NOT what you want.  1.  It increases drag.  2.  It often indicates problems - leaking seals, internal pipe leaks, corrosion in exposed flap tracks etc.

Ok, I do not find references to underpin that statement, but a gifted car mechanic once told me, that white lacquer is harder than other colors. (Well, most craftsmen in Germany have white cars...).

The Concorde was painted with a special type of white paint because any other color would cause the airframe to heat up way beyond its design limits at Mach 2 cruise. The Pepsi Concorde was painted blue, and was hence not allowed to fly at Mach 2 for more than 20 minutes at a time: http://www.concordesst.com/history/events/pictures/pepsi6.jpg

The 747 in your second photo is an outdoor exhibit at a museum: it's not at all representative of the condition of an in-service commercial airliner.

@DavidRicherby Don't kid yourself... https://i.sstatic.net/xjmrM.jpg (that, just from searching for 'dirty plane')

I would be asking why Air France planes are so dirty based on your findings!

I'd say everything painted into white does reflect most light, thus protecting the object under from heating, and this applies for Airplanes, Cars, and Buildings.

Many aeroplanes used to be raw silver metal. White makes them look more modern for their owners. Silver is more efficient. Here is a boeing being repainted in time-lapse: https://www.youtube.com/watch?v=rBcjhqWigK8

@ufomorace Could you expand upon Silver is more fficient. Do you mean weight efficiency or reflective efficient? BTW. aesthetically, I actually like raw silver metal the best. It's my favorite color, and since it's so shiny it's kinda like 2 colors in one: white and light gray. Maybe you could add some colored streak highlights, but there's nothing wrong with silver IMO.

Hi, the trade description for a silver aeroplane is called "polished", so if you check "polished aeroplane" on google there are many very cool photos of silver aerplanes. Perhaps they have also been painted chrome. To polish aluminium i think that you need to buff it with a fluffy buffer quite often because it oxydizes and tarnishes, so it is roughly the same but slightly more expensive that paint for a fleet. here is an article on it: polished airplane by Boeing. http://www.boeing.com/commercial/aeromagazine/aero_05/textonly/fo01txt.html

White has some significant thermal advantages over color. In most cases, this is probably just a benefit in terms of keeping the cabin slightly cooler. In the case of 'plastic' airplanes (those built with composite construction), some airframes require the use of white paint on upper surfaces to keep some elements within limits. Early Diamond Aircraft designs had a 38° C outside air temperature limit, past which the main spar is not considered structurally sound.
As a result, Diamond would only manufacture aircraft with predominantly white upper surfaces, as darker surfaces could result in significant heat gain.

Avweb has an article on the DA-20 that shows the bright red over-temp indicator that raises the maximum temperature to 55° C, added as a Service Bulletin.

I read some time ago that supersonic aircraft like Concorde also required almost all-white livery due to thermal issues

It's worth noting that I doubt this is the definitive answer, even though it's been accepted. The structural temperature issues are only really relevant for the newest composite designs. There's no place on earth that's likely to be hot enough to cause issues with an aluminum airframe.

...except maybe a volcano.

@egid:  How about at 80,000 feet and Mach 3.2?  The SR-71 Blackbird was built out of titanium because the high temp "enjoyed" at cruise speed would melt aluminum easily.

Also, the SR-71 was black.  It was heated by friction with the atmosphere, not by solar heating.  Diamonds as noted above are white to avoid getting solar heat, the SR-71 was black to efficiently radiate the heat of friction away.

@SkipMiller I'd say it's the exception that proves the rule.

@egid:  Without question, the SR-71 is exceptional in many ways!

The green coating is a primer (historically zinc chromate, these days zinc phosphate is used as it's less of an environmental hazard) - While you could theoretically fly around like that it is a rather ugly shade of green, so most folks (and airlines) tend to paint their aircraft some other color.
So why white?
White has a few advantages, but the two biggest ones I can think of are these:

White is a nice, neutral base color.
Artists typically start out with a white canvas, or white paper - it's a color associated with "blank" and "clean" (the latter moreso if you actually wash the aircraft, of course...).
Starting with a white base coat allows the airline's marketing department to paint whatever livery they want on their flying billboards.
White doesn't fade.
Or perhaps it's more accurate to say "you don't really notice when white fades.".
An average airliner will have several paint jobs during its service life, but the longer you can go between having to reprint the plane the better, and if you don't have to worry about the paint fading and looking old for a good long while you cans stretch the time between paint jobs without having your passengers start making snide comments about your fleet of flying hoopties.
All colored paints will eventually fade from exposure to sun and the elements, particularly exposure to the sun at 30,000 feet, where substantially more UV radiation is hitting the paint and accelerating the fading process.

Of course there's nothing that says the base coat HAS to be white: Southwest has a very attractive blue/red/orange livery, and American Airlines has traditionally incorporated polished aluminum in their livery (which also affords a substantial weight savings, by not priming & painting the entire body of the aircraft).

You say "a substantial weight savings" - Do you have a reference for how much the paint weighs for a given plane?

@RichardMiskin It obviously varies depending on how much surface needs to be covered, and to a lesser extent on the type of paint used (urethane paints are pretty much the standard in aviation these days).  This Daily Apple article has a table from Boeing, which puts the weight of a full paint scheme on a 747 at 555lbs (versus 55lbs for  polished skin & a logo).

@vortaq7 There was an article in the seat pocket magazine years ago on an American Airlines flight.  Their former livery - polished aluminum accented by a stripe of multicolored paint - saved 200 pounds.  that is from memory and applies to the MD-80 series.  So they save the weight of one passenger on each flight.  I wish I could remember the fuel savings they claimed, but it was substantial on an annual basis.

"Ugly shade of green"... you do realize it's St Patricks Day, and you've just antagonized everyone of Irish descent? ;) I seem to remember a story about interned B-29s that landed in Russia during WWII due to battle damage over Japan, and the Russians copied them exactly, down to the exposed green primer in the aft portion (the factory had run out of interior white paint, or something).

@SkipMiller Unfortunately the cost savings from not painting the aircraft is more than offset by the cost to keep the aluminum polished.  :)

@RichardMiskin Check out this YouTube clip from Megafactories, with Boeing painting a new 747. This would concur with voretaq7.

@Lnafziger:  Your source for this information?  If that is true why did they do that?  Doesn't make sense!

@SkipMiller My source is Boeing and the reason is that it ends up being a relatively small difference and is a marketing decision by the airline.

The 737-700 is 179 lbs. to paint the entire aircraft.

@Lnafziger The amount quoted for the 747 in the above clip is 500kg, so 1,100 lbs.

@DannyBeckett My number is for the 737 (which is what voretaq7 was referring to in his answer where he said "significant weight savings"), and not the 747.  According to the document that I linked to in my previous comment on Boeing's website, the 747-400 paint weighs 555 lbs (251.7 Kg), so I'm not sure where they got 500 kg.

@Lnafziger Sure, I was just pointing out that the weight savings could be even higher. I'm not sure where they got that figure from though.

+1 for fleet of flying hoopties!

As noted in my answer below, (though it was a while ago and I forgot to mention it here) the green shown on the first picture in original question is NOT primer, it is Temporary Protective Coating, a spray-on plastic that is chemically rinsed off during the paint process.

Of course, in certain situations a plane has to be painted white...or another colour.
Concorde had to be finished in a special, highly reflective white paint to mitigate the extreme heating effects that friction caused at mach 2.

[It] reaches 127° C at the nose and trailing edge, but the special ‘high-reflectivity’ white paint helps reflect and radiate heat. Its reflectivity is 80 out of 100, compared to the rating of normal white paint of 45-50 out of 100, Concorde is re-painted every three years.

Conversely, the SR-71 Blackbird was painted matte black for almost exactly the same reason, plus some radar signature reduction.

That Concorde-link gives a 403. :-(

@MartinSchröder It seems to be working now, but you can always view it at https://web.archive.org/web/20130713084122/http://www.speedbirds.pwp.blueyonder.co.uk/concfacts.htm

Reflective paint makes the plane a poor radiator of heat.

Two reasons for white I've been told many times are:

white paint is cheaper than other colours (either because it costs less and/or weighs less)
white makes it easier to apply names and logos of other companies when leasing your aircraft to someone else. Saves a trip to the paint shop, instead you just apply stickers.

Both make sense, but whether they're the main reason I don't know.

Do they really use stickers for names and logos? I would think it likely they would come off too easily.

@PhilPerry yes, they do. And yes, they do sometimes come off when not properly applied. But it's quite possible to use stickers that require special chemicals to get off.

Why couldn't you place a sticker on an airplane with some other color?

@Speldosa you can. White is however nicely neutral. If you put KLM stickers on a US Airways aircraft... Now put them on an Air France aircraft... In both cases after removing the original titles and logos of course. And that's how rental/short term lease companies work.

Stickers are also used for special, non-permanent liveries, such as the Hobbit-themed liveries that Air New Zealand has had the last few years.

@reirab of course, same thing effectively as a short lease or rental. You put the styling on for a very limited time only.

Contrary to voretaq7's highly upvoted answer, the green coating in your first image is NOT a primer. It's a spray-on plastic protective coating usually referred to as "Temporary Protective Coating (TPC)". This is used primarily as a barrier to oxidation, as there is a very thin layer of very pure aluminum on the exterior surface of the skin.
During some of the videos available of the painting process for a new aircraft, you will see them spray a chemical over the entire airplane as the first step. This chemical dissolves the TPC, and leaves a super-shiny aluminum surface for them to then proceed to paint.
The primer paint used on the exterior of the skin during the full paint process is usually a yellow color, not the traditional "Boeing green" seen on interior parts that don't get a final paint coat.
As for why white is the primary color of choice, I unfortunately do not know. However I would point out that Southwest Airlines does not use a white base-coat on their current livery (term for the paint scheme), American Airlines' new 787's are actually a very light blue, and there was the Air New Zealand black 787 shown in another answer...
Source/Disclaimer: I am employed by Boeing on the Commercial production lines.  Information I have provided  is, to the best of my knowledge, accurate and not considered proprietary.

Where is the "there" that you work?

Boeing, commercial aircraft, structures assembly

Welcome to Aviation.SE. I hope you stick around, with that background, your knowledge will be highly prized around these parts!!

@Rayanth, that makes sense. I live very close to many boeing factories and I have noticed that the planes are yellowish-green. You are right, they do not look like boeing green, they instead look more of a yellowish green.

Another minor reason is that white color protects from adverse radiation better.
In regards to nuclear bombers this is called 'anti flash white', though regular planes flying at high altitudes supposedly benefit from this as well.

White makes it easier to spot another plane in the air, since white shows up well against the ground.  This is why many early planes were colored on the bottom half and white/silver for the rest.
Of course, this matters much less with modern IFR equipment and doesn't matter really at all with high-altitude jets, but old habits die hard, I guess.

is this also the reason why military planes are not white?

@user13107 That's probably more to do with stealth/camo: Dark spot under cockpit on A-10s

@DannyBeckett that sounds like exactly what user13107 is talking about. :) If white makes an airplane easier to spot, camouflage (not white) would make military planes harder to spot.

I'm not so sure about white being easy to spot - at least in my personal experience. The most visible plane I've ever seen was painted an utterly hideous shade of yellow-green (about the same color they paint the ARFF trucks) - I saw that thing buzzing away in the pattern when I was about 8 miles from the airport and never lost sight of it!

@user13107 White is actually pretty hard to spot when looking up at it, particularly if there are clouds. This is why some airplanes have white tops (easier to spot with the ground as a backdrop) and dark colored bottoms which stand out from the sky.

And consider that a lot of birds and especially fish do it the other way around, lighter underneath ;-)

@SteveJessop, yeah, countershading is very effective. That's partly why military aircraft are often camo on top, lighter solid underneath.

I agree with @voretaq7, a shade of yellow or yellowish green would be easiest to spot. If you have ever seen a DHL plane you know what I am talking about. Those things are painted yellow AND red. I think that the reason most planes are painted white is that you can put a lot of logos on white, and that white looks cleaner than other colors.

I work for a printing company.
Think of this - why everything is printed mostly in black ink? why not navy blue, or dark hunter green?
Because black is a stock color.
The manufacturer stocks it, and uses it left and right.
Most other colors have to be a special order - this comes with increased costs, lead times and paint expiration. If you did not use all of it, paint does expire.
To the printer-happy community let me remind you that painting a jet is not like printing a page where three base inks are used to create a perception of color.
Paint of any color has to be manufactured separately.
So I think that from a manufacturing standpoint it is just so much cheaper to use stock white paint.

Most of the above reasons are valid. I worked at Eastern airlines and we switched from the white/blue livery to polished aluminum because the weight/drag reduction vs. increased manpower cost still was overall cost effective. Remember a 747 uses around 90 gallons of paint weighing in the vicinity of 550 pounds.

I worked at Eastern airlines, wow, that's something you just don't hear every day anymore!  My uncle worked for them until they shut down.

of course now, with aircraft ever more made of plastics and other non-metalic components, having them in bare metal requires painting them that way, which is costing American Airlines a fair bit :)

You are all right to a point / close:
Here are the main factors in order of importance:

White paint is both cheap and easy to apply.
White paint on most of the aircraft allows for quick and easy    resale (the new airline simply puts new tail art on it).
White paint shows any anomalies quickly — hydraulic/fuel leaks,    dirt, damage.
White paint has the best thermal reflective properties for both    radiation from the sun and reflected radiation from the
tarmac    (although in northern climates / winter this is reversed)!

People don't generally choose their airline because of it livery, they generally choose the cheapest fare. Therefore white paint is the colour of choice. It all boils down to money.

could you substantiate your claims? what makes white paint cheap/easier to apply (!) than a color paint? Also, please consider polishing language and spelling.

Apart from technical reasons, I am not really aware of, let me put it this way: Would you fly in a black plane? Probably not.
You associate colours with objects, feelings, situations etc.
White can be trivially associated with sky, just like blue. I believe most airlines stick to these colours, since it helps associating the brand with sky. But the colours can have deeper meaning which is often used for marketing purposes. White is often associated with trust and perfection. Blue is associated with quality, security, trust or business. Notice many company logos are blue and white - Facebook, IBM, GE, Samsung, Hilton.

Actually this black plane looks pretty slick: https://i.sstatic.net/5oUfl.jpg

I must say this really surprises me and I was not aware of that.

To be fair, this has special meaning in New Zealand.  Their national rugby team is named the All-Blacks, and Air New Zealand as a part of their sponsorship of the team has adopted the black livery for a few of their planes.

Though personally, I'm quite a big fan of this black plane:  http://upload.wikimedia.org/wikipedia/commons/4/47/B-2_Spirit_original.jpg

Do you have any sources for this or is it just your personal opinion?

The reasons for the colors to be used is just my opinion, however the color associations are not made by myself, you can see for instance http://www.joehallock.com/edu/COM498/associations.html. I just corrected few inaccuracies.

White paint won't absorb as much heat as black or another color, imagine walking on bare blacktop vs. the white painted line.  I found this article to save me the explanation.  The black paint absorbs the entire spectrum of photons, whereas the white reflects most of the  photons.  When a photon is absorbed, its energy is absorbed (a photon is energy) and the heat of the plane increases.

It's quite simple actually: White paint is both cheaper and lighter than paint with colorant added to it.  Both of these are important when trying to reduce cost at an airline!

I'm led to believe cost isn't the only reason... Something about white reflecting heat? airliners.net answers.yahoo.com

I'm not sure what the weight differential is - when I go in for my paint job I'll have to pick some brains on that. I imagine the difference isn't too substantial, but on the scale of an airliner an ounce or two per gallon would add up to quite a bit of weight :-)

@DannyBeckett, Surfaces that appear white appear so because they reflect all wavelengths of light. Surfaces that appear black appear so because they absorb all wavelengths of light. Other colors are in between. When a material absorbs light energy, it gets transferred into heat energy. I'm not sure that this is the primary reason it's commonly used on aircraft, but I'm sure it does help.

Are you sure white is lighter? Coloured paint is not white paint with a colour added: white paint is typically itself coloured with titanium dioxide.

white paint, itself, has a colorant added to it. Most truly white paints use titanium dioxide as a colorant.

Rather than painted white, Boeing documents the option of keeping a fleet "polished" raw aluminium. Polished is more efficient to fly, but it requires more maintenance, buffing the entire plane multiple times a year, whereas painted aeroplanes require maintenance every 4 years... Here is an article with silver colored versus painted aeroplanes by Boeing:
Polished vs Painted by Boeing
here are pictures of polished aeroplanes
There are various fan pages that talk about polishing planes.
Military planes are often painted different from civilian ones and with regard to camouflage, grey, cammo, blue for sea, it's mostly fleet themes and standards.

Paint on a large plane adds something like 230 kilos, 475 lbs for a B777-200 airliner. Amercian airlines was the last to have silver aeroplanes: http://www.airlinepilotchatter.com/2013/01/new-american-airlines-livery-to-paint.html

Welcome. The question is: "Why are most painted white? Surely a darker colour would hide dirt better". Your post doesn't seem to address that.

Please read the above answers and apply the same comment to those which you feel apply, it will be many of them, as none mentions cleanliness and some mention polished being the best color for an aeroplane rather than using paint.

As stated before,  White is cheaper.  There is no thermal benefit for subsonic planes at cruising altitudes.  Windchill and ambient temperature at flight levels are below freezing.


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Would TCAS ever issue a "climb, climb!" RA that would stall a jet with a low airspeed?
As a for instance, let's say there are 2 jets going at low airspeeds (close to $V_S$) that get too close to eachother and a TCAS Resolution Advisory callout is announced.
Would TCAS issue an RA to order one of the jets to climb, resulting in loss of airspeed and a stall?

The FAA publishes Advisory Circular 20-151A, which says in section 2-17(a):

Because TCAS II can only accept a limited number of inputs related to
  airplane performance, it’s not possible to automatically inhibit CLIMB
  and INCREASE CLIMB RAs in all cases where it may be appropriate to
  inhibit such RAs. In these cases, TCAS II may command maneuvers that
  may significantly reduce stall margins or result in stall warning

Seems like a "hard left, hard left now!" RA would be good to have in this situation...

@DannyBeckett: In a turn you have higher wing loading and therefore higher stall speed. So, no, it probably wouldn't.

@DannyBeckett: Of course the main reason why there is no turn left/right advisory is that the current TCAS is based on Mode C transponder and that only encodes altitude (and distance is calculated from response time). When they switch to ADS-B (that contains speed, heading and position), it will become possible.

It should be noted, though, that in the hierarchy of command maneuvers, warnings, and aural alerts, TCAS generally ranks lowest in order of priority. In other words, if EGPWS is issuing a "Pull Up" alert, it may suppress a TCAS RA to rapidly descend. Obviously, this sort of situation would be pretty strange, but I do recall there being an entire hierarchical list of all caution and warning systems that have priority over one another. TCAS was pretty far down on that list.

@JanHudec: Turning RAs were proposed and rejected over 25 years ago. The reason they were discarded is that vertical RAs create the needed separation faster than a turn. It takes time to get enough roll angle to get the turn rate up and lateral separation means you need more distance to accommodate the wing span.  Also, all TCAS II systems use Mode S transponders which are capable of 25 foot altitude reports and ADS-B out.  ADS-B is used by newer TCAS, but not in the RA calculation. It's used to reduce interrogations of distant aircraft. TAs and RAs are all based on interrogation data.

There is no more risk of stalling an aircraft from a TCAS alert than from being instructed verbally by ATC to climb to a higher altitude for traffic avoidance.  Or, (in the case of slow flight at approach speeds) to execute a go-around from an approach to land.
An aircraft can only be stalled if the critical angle of attack is exceeded.  A TCAS alert to climb is just that, it does not command the pilot to do anything other than climb to a safe altitude.  It tells the pilot where to go, but not how to get there.
Initiating a climb at low speed does not automatically result in a stall as the question would seem to imply.  Proper pilot technique is to add power and adjust attitude to maintain safe airspeed and AOA.  The aircraft can only be stalled if the pilot pitches the nose up, fails to add sufficient power in the climb, allows airspeed to decay, and holds the attitude until a stall is reached.
So, the short answer is no.

Welcome! This is an interesting remark about pilot role, but regarding the question, this doesn't add to the selected answer. This should be a comment, not an answer.

Thanks, and I'm sorry if I didn't fully understand the ROE regarding answering a question vs commenting.  I attempted to post to comments instead but was told I needed a credibility rating of 50 to do so.  (odd that I can answer the question but not comment?!)

Anyhow, I would have simply deleted the post but I don't agree with the answer.  (I made a minor edit about proper pilot technique for climbing an aircraft for clarity)  While technically correct, the answer does not consider pilot techniques.  So, feel free to delete my post, but I have 20 years of military flight experience +ATP.  Thanks.

Don't worry about not being able to comment now, we know, keep answering and your reputation will be sufficient very quickly.

Thanks.  My original "answer" is worded more as a response to the previous answer, which I felt was lacking.  Still not totally sure how this reputation thing works, but I would be happy to draft a better response to the original question if you felt that replacing Cpast's answer was appropriate.

Or re-edit the existing one and let people vote with the up/down arrows?

You can edit your question as many times as you want, but keep in mind this needs to be an answer to the original question (so you are competing "against" existing answers). For criticizing an existing post without fully answering, you can only comment. Users will vote if you get it correctly, each vote is 10 reps, so 5 votes and you can comment any post. There are also many questions that don't have yet an answer.

It's a reasonable answer IMHO. I upvoted it.

Same here.  The pilot role is a very important part of this and can't be discounted.

Thanks Lnafziger.  The way I read the original question it's kind of like asking "would your car's GPS ever command you to turn into the path of an oncoming truck?"  Yes, it might, but as driver you are expected to exercise sound judgment.

I think this is possible. TCAS generates Resolution Advisories based on various factors, and two aircraft equipped with TCAS transponders then coordinate their RAs. Each aircraft knows only the bearing, altitude, and distance of the other aircraft; it gets its data from Mode S transponders, which don't send things like stall speed or maximum climb/descent rate. That's all that the basic TCAS algorithm uses. It doesn't necessarily even know its own plane's performance information.
Separate from the basic RA system, an installation can be configured to not issue "Climb" or "Increase Climb" RAs under certain circumstances (each TCAS RA is sufficient to avoid a collision if the other aircraft does nothing, so "Maintain Climb" or "Reduce Descent" or "Do Not Descend" may be enough). This doesn't change the general sense of an RA (climb/descend), but rather modifies the actual RA issued to its own aircrew. However, it does not have to be configured in such a way, and the configuration won't necessarily catch all stall situations. So a TCAS system is certainly capable of issuing an RA that will stall your plane.

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Who decides an aircraft's maintenance schedule?
I've found a 737 maintenance planning data document produced by Boeing, which gives a suggested schedule on page 6.
I've also found a British Airways Fact Book document, which on the last page gives a schedule.
So who decides how often an aircraft and its individual components should be inspected? Is it the manufacturer, the airline, or somebody else? Is there a set standard or does this differ between manufacturers/airlines/countries?

The manufacturer lists all maintenance that should be done, and when. This is the first document you found, from Boeing. Some of this maintenance may be required, other things might be recommended.
The operator can use that plan, while following maintenance laws (e.g. the required annual inspection). Or, they can submit a new rolling maintenance plan to their National Aviation Authority (e.g. FAA) for approval, which is usually done to prevent large downtime events for the aircraft. (This "rolling inspection" is done so that all of the necessary things are inspected at the proper intervals, but not all at the same time.)
There's a bit more, too: the Aviation Authorities can create Airworthiness Directives that can add additional required maintenance that must be done within certain limits (usually hourly, but can be situation dependent) for the aircraft to be airworthy.

Just came across this post I realize it's a bit dated but the information given here is a little incomplete.
The OEM sit the initial intervals for most maintenance items.  If you are a 121 certified Air Carrier with an approved Reliability program, Operation Specification D074, then with the proper analysis you can adjust the intervals for those tasks not governed by regulation, such as Air Worthiness Limitation items (AWL's).

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Why is the de-icing fluid sprayed on jets coloured orange?
On a cold night or if it's been snowing, jets are sprayed with orange de-icing fluid:

But why is it orange?
If it's to see where's been sprayed and where you've missed a bit, black would be the most contrasting colour on white; while yellowy-green is the strongest/most visible colour to the human eye; and blue is the least affected colour for colour-blind people.
Just curious as to why orange.

Here is a white one: http://imgur.com/kB05aNI

Given the answer, this is no way off-topic. Who keeps flagging questions with no criteria?

There are actually two types of de-icing fluid, one is orange and one is green. According to a product safety sheet for one brand:

The dye is intended to indicate which parts of the  aircraft have been
  treated and to differentiate between fluids (Type I fluids are orange,
  Type IV  fluids are green).

As to why orange and green, presumably they aren't easily confused with other liquids used around aircraft. Black would look like oil or another fluid if it pooled on tarmac, hydraulic fluids or oils are often yellow/green in colour and blue is a very common colour in aircraft painted liveries. So I guess that orange and green were sort of the ones 'left over' after all the other obvious colours had been considered.

Here I found there are 4 types: http://www.boeing.com/commercial/aeromagazine/articles/2010_q4/2/

@HorstWalter: Yes. Notice that the two types in the answer are numbered one and four. The two in between seem to be obsolete.

Supposedly Type IV is favored over Type II now, and Type III is for slower aircraft.

About your question on why black is not used even though it provides the highest contrast to white: thats not practically true. The problem is, that black and white have the same hue and color saturation (or rather they have no color saturation, and thus the hue is undefined in both cases), so they only differ in brightness/luminance. And a difference in brightness can easily be caused by different lighting conditions.
So for black-dyed de-icing fluid, you couldn't reliably tell whether a black-looking spot has been de-iced, or if it is just not well lit. Similarly, a white-looking spot may not have been de-iced, or may just accidentally reflect more light that its surroundings (e.g. due to specular highlights, retro reflectors, different incidence angle to the light source, or different surface reflectance), Colored fluids solve that problem, as color (e.g. green dyed liquid vs. white snow) barely changes under varying lighting conditions.

As an additional note, black does not contrast strongly with blue, and under low-light conditions, does not contrast strongly with red.  White is the dominant color of airplanes, but hardly the only one.  Orange, on the other hand, is rare.

lol - so Easyjet aircraft cannot be safely de-iced?


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Does the "Climbing right turn" instruction of this SID violate the "Climb to 400 feet above DER before turning" requirement?
I was examining the LaGuardia Five SID, and I noticed that for runway 13 (Coney Climb) it gives the instruction, immediately after takeoff, "Climbing right turn heading 180 degrees to intercept CRI R-043" (textual description of this SID can be found here).
However, as far as I have understood, performing a turn before covering a 400 feet vertical distance above the Departure End of Runway point is not allowed (in other words, turning before covering the Initial Climb Area). Is it implied that the pilot should not perform this turn before meeting the particular requirement?

I may be missing something, but where does it say "immediate right turn" in the SID? I only see regular climb instructions, and instrument departures are always assumed to start with a 400' straight-out climb anyway (see the IPH, Chapter 1). What makes you think that the SID requires you to turn before reaching 400'?

@Pondlife thank you for your comment, I had forgotten that this is clearly stated in the Instrument Procedures Handbook. Please feel free to post this as an answer so I can mark it as the accepted one.

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


text:
How do procedure designers calculate Minimum Enroute Altitudes?
When the designers of instrument flight procedures design a new procedure, I understand that they use certain manuals, such as ICAO's Doc29 or FAA's  TERPS and PBN (8260.58), in order to construct each leg of the procedure by taking into consideration the required obstacle clearance. I can see how one could deduce the Minimum Obstacle Clearance Altitude from the calculated Obstacle Clearance Surface, but how do they know which altitudes, based on each leg of the designed procedure, provide adequate reception of relevant navigation facilities and ATS communications? How do they get that information?


text:
What is the operational advantage of starting an RNAV SID with a VI-CF leg combination?
I am examining Standard Instrument Flight Procedures, mainly from FAA's charts and Coded Instrument Flight Procedures ARINC-424 file (CIFP). I am keen in understanding the operational advantages or other criteria by which a procedure designer would choose a certain leg combination over another.
As far as I could tell by searching through the aforementioned CIFP file, the following leg-combinations are mostly used for the first couple legs of an RNAV departure procedure:

VA (Heading to Altitude) -> DF (Direct to Fix)
VI (Heading to Intercept) -> CF (Course to Fix)

The first leg can also be DF, CF or TF, as it is stated in FAA's 8260.58A Performance Based Navigation manual, in Chapter 5. Departure Procedures, section 5-1-1. General.
For an example, take BRYCC4 Departure and its respective narrative page. Takeoff from runways 16L/R uses a VA-DF leg combination ("Climb heading 173° to 5934, then right turn direct to cross MUGBE at or below 10000..."), whereas takeoff from Runway 17L uses a VI-CF leg combination ("Climb heading 173° to intercept course 127° to cross GISTT at or above 7000...").
I can understand the need for a VA leg before a turn, since the aircraft is required to climb to a certain altitude (eg. for LNAV engagement or for obstacle clearance) before the first turn. However, I have a difficulty grasping the circumstances under which a VI leg is a more preferable choice.
Could someone point me to any source material that describe the criteria on which the decision about the leg combination during takeoff is based?

Could it just be that having a navaid in the right location allows a VI-CF, whereas not having one leaves them with only VA-DF available?

Well with an RNAV SID the fms or gps can generate direct to or a radial or course to any fix as necessary or described in the procedure so that wouldn't have any bearing.

The PBN manual is the place to start. In Section 5 on departure procedures it includes the following reference:

b. Leg type limitations. See Order 8260.46, paragraph 3-1-5 for
  permissible leg types.

Also, all the TERPS requirements also apply, except where modified by the PBN manual.
There's usually no one rule that applies universally. It's about finding the best solution that meets the terrain/obstacle avoidance and meets the necessary airspace and air traffic needs.  I can't say with certainty why the procedure writer made the choices they did, but I can point out a few things that would apply in the SID you reference.
The 17L/R departure is primarily constrained by the 7000 ft minimum at GISTT. It's there to keep the path within the Class B airspace.  There's a cutout with a 7000' floor around Front Range airport to the southeast. GISTT is located at the edge of the cutout.  It's far enough south to leave room to reach the minimum altitude at the published minimum climb gradient.  The VI/CF is the solution that gets you to GISTT expeditiously.  It also has an intercept angle of about 50 degrees which works well with the FMS and LNAV steering.
The 16L/R departure has two factors that push it to the VA/DF solution.  First is that it departs over runway 07/25 and there's no intercept (more later), the altitude termination defines when the turn can begin. The reason the DF to MUGBE is used instead of a CF is that there is a 140 degree course change.  You can't have a VI/CF combination with that large of a course change. Thus the VA/DF combo.  If MUGBE (or another convenient waypoint) was better located, a VA/RF leg combo could work but RF legs are relatively 'new' in the RNP FMS world and they aren't very common.

Maybe I am not grasping all the nuances here, but it seems pretty obvious to me that if obstacle or terrain clearance is required you would design a departure procedure to maintain a certain heading until clearance is assured, (VA) then route from there.  If obstacles or noise abatement is not an issue, then the earliest possible turn to intercept an airway, (VI) or proceed direct to a fix on the intended route is a more efficient path.  This would allow the aircraft to turn on course as soon as possible.

This. The VI leg means that your groundtrack to the fix is always the same, rather than varying depending on how quickly or slowly the aircraft climbs. When that precise, repeatable  groundtrack is desired (and the terrain permits), the VI is what the designer can use to get it.


text:
When helicopters are instructed to air-taxi direct to an airport location, are they allowed to pass above other aircraft and people?
When helicopters are instructed to air-taxi direct to a location at the airport, are they allowed to pass above other aircraft / people / road traffic?

The FAA helicopter flying handbook says "it is expected that the helicopter will remain below 100 feet AGL with an appropriate airspeed and will avoid over flight of other aircraft, vehicles, and personnel," which would make it inadvisable to do so, whether or not it's allowed.

In my experience, the helicopters stick to the taxiways and their ground clearance is similar to that of other fixed wing aircraft. They are only cleared if the taxiway is clear and allows them to reposition without conflict.

@Zach - that looks like it should be an answer (for the FAA's realm, anyway) rather than a comment.

No. A helicopter taxiing has to follow the exact same rules as any other taxiing aircraft.


text:
If an approach chart says "GPS required", do I need GNSS capability to fly it?
If an aircraft is /Y, /C, or /I (RNAV no GNSS), can they fly an approach chart that says "DME/DME/IRU or GPS required"? GPS in my mind is a GNSS. If you don't have DME/DME/IRU and are RNAV (non-GNSS) capable, can you legally fly the approach?
An example of a chart that has the "DME/DME/IRU or GPS required" notation is here.


text:
Do IFR / VFR weather minimums include ceilings?
I'm not really sure how to ask my question, but there are classifications for weather conditions for MVFR, IFR, and LIFR. The FAA also has weather minimums for flying VFR in the different airspaces here.
What is the difference between when we would use one over the other? If I am departing from an airport in class bravo airspace, would I need three statute miles of visibility and remain clear of clouds (second link), or would I also need to adhere to the minimum VFR ceiling of 1000' (first link)?

FWIW, the two links you posted are talking about different things. The first tells you how weather conditions are classified for display in weather reports. The second tells you what cloud clearance you need to maintain while airborne. So it doesn't make much sense to compare them directly. I don't know if that's useful, but maybe it can help you to narrow down what you're trying to ask.

There are two FARs that govern takeoff. The first is §91.155 and quite clearly states that no one may take off under VFR (except at Class G airports) if the ceiling is less than 1,000'. You could ask for a Special VFR, but you won’t get it at Class B—in fact many (most?) have a notation on the chart saying that it is not allowed.
The second FAR is §91.175 (f) governs visibility in IFR operations and does not apply to Part 91 operations (which I assume you are).
So assuming you want to depart VFR, you would need 1,000' ceiling to depart. If you want to depart IFR, you can do so with 0 ceiling and 0 visibility. (Subject to any conditions in the Obstacle Departure Procedure §91.175 (f)(3)).

§91.155   Basic VFR weather minimums.
(c) Except as provided in §91.157, no person may operate an aircraft
  beneath the ceiling under VFR within the lateral boundaries of
  controlled airspace designated to the surface for an airport when the
  ceiling is less than 1,000 feet.
(d) Except as provided in §91.157 of this part [Special VFR—which you
  won’t get at a Class B airport], no person may take off or land an
  aircraft, or enter the traffic pattern of an airport, under VFR,
  within the lateral boundaries of the surface areas of Class B, Class
  C, Class D, or Class E airspace designated for an airport—
(1) Unless ground visibility at that airport is at least 3 statute
  miles; or
(2) If ground visibility is not reported at that airport, unless
  flight visibility during landing or takeoff, or while operating in the
  traffic pattern is at least 3 statute miles.
(e) For the purpose of this section, an aircraft operating at the base
  altitude of a Class E airspace area is considered to be within the
  airspace directly below that area.
§91.175   Takeoff and landing under IFR.
(f) Civil airport takeoff minimums. This paragraph applies to persons
  operating an aircraft under part 121, 125, 129, or 135 of this
  chapter.
(1) Unless otherwise authorized by the FAA, no pilot may takeoff from
  a civil airport under IFR unless the weather conditions at time of
  takeoff are at or above the weather minimums for IFR takeoff
  prescribed for that airport under part 97 of this chapter.
(2) If takeoff weather minimums are not prescribed under part 97 of
  this chapter for a particular airport, the following weather minimums
  apply to takeoffs under IFR:
(i) For aircraft, other than helicopters, having two engines or less—1
  statute mile visibility.
(ii) For aircraft having more than two engines— 1⁄2 statute mile
  visibility.
(iii) For helicopters— 1⁄2 statute mile visibility.
(3) Except as provided in paragraph (f)(4) of this section, no pilot
  may takeoff under IFR from a civil airport having published obstacle
  departure procedures (ODPs) under part 97 of this chapter for the
  takeoff runway to be used, unless the pilot uses such ODPs or an
  alternative procedure or route assigned by air traffic control.

The FAA does not define "LIFR" or "MVFR"
These are not terms that the FAA makes reference to.  Go look in the FAR/AIM, section 1.2.  You'll find VFR defined, and IFR defined, but not the other two.  Why is that?
Because those are terms used by the National Weather Service, not the FAA
That means that if you want to operate an aircraft, you obey the FAA weather minumums that are provided in your second link.
The information in the first link is about reading weather reports.  As far as the FAA is concerned:

VFR is VFR
MVFR is also VFR
IFR is IFR
LIFR is also IFR

So answer my question
Ok.  If you're departing from an airport, and you will be in class B airspace from the moment your wheels leave the ground, then as long as you have 3 miles visibility and you don't touch a cloud you can fly.
However, it's not likely that you'll be in class B airspace from the moment you leave the ground unless you're departing from the main airport (LAX, ORD, and so on) and if that's the case odds are you're on an instrument flight plan anyway.

Weather minimums for VFR are based solely on airspace type, time of day and altitude and they do have ceiling requirements and visibilities associated with them (see §91.155 (a)).  These represent the minimum meteorological requirements which must be met for a VFR flight to be conducted at that particular location and air space.
Surface weather, however, may be reported in terms of VFR, MFVR, IFR and LIFR as follows.

VFR conditions - ceilings > 3000 ft AGL and 5 sm visibility.
MVFR conditions  - ceilings between 1000 ft and 3000 ft AGL, visibility 3-5 sm.
IFR conditions  - ceilings 500-999 ft AGL, visibility 1 sm but < 3 sm.
LIFR conditions  - ceilings < 500 on ft AGL, visibility < 1 sm.

An airport may be reported with surface weather in one of those categories if either the ceilings and or visibilities meet the criteria.
Note that there may be situations where a pilot can legally depart VFR in surface weather reported to be IFR eg a pilot departs VFR at an airport in class G surface airspace in daylight with a surface visibility of 2 sm.  This is legal, as it still meets the VFR weather minimums for that airspace but is considered IFR surface weather.
In the example which you sided, ie VFR departure into Class B, you must meet the VFR weather minimums in order to operate in that airspace ie 3sm visibility, clear of clouds.
HOWEVER
§91.155 (c), (d) requires that no aircraft depart an airport in controlled airspace unless the surface conditions are reported as ceilings are 1000 ft AGL or greater AND the visibility is greater than 3sm.
Therefore 1) reported surface weather MUST meet the minimums listed in §91.155(c), (d) prior to takeoff under VFR into controlled airspace and 2) once airborne, must comply with the weather minimums listed §91.155 (a) while airborne.

Sounds like you wish to depart VFR.

would I need three statute miles of visibility and remain clear of clouds (second link)

Is that Special VFR? Special VFR might be a little less restrictive even.  You can request Special VFR, but it will never be offered (explicitly) to you from ATC.
The Special VFR requirements, to be complete:

14 CFR 91.157 - Special VFR weather minimums. eCFR Authorities (U.S. Code) Rulemaking What Cites Me prev | next § 91.157 Special VFR weather minimums. (a) Except as provided in appendix D, section 3, of this part, special VFR operations may be conducted under the weather minimums and requirements of this section, instead of those contained in § 91.155, below 10,000 feet MSL within the airspace contained by the upward extension of the lateral boundaries of the controlled airspace designated to the surface for an airport.
(b) Special VFR operations may only be conducted -
(1) With an ATC clearance;
(2) Clear of clouds;
(3) Except for helicopters, when flight visibility is at least 1 statute mile; and
(4) Except for helicopters, between sunrise and sunset (or in Alaska, when the sun is 6 degrees or more below the horizon) unless -
(i) The person being granted the ATC clearance meets the applicable requirements for instrument flight under part 61 of this chapter; and
(ii) The aircraft is equipped as required in § 91.205(d).
(c) No person may take off or land an aircraft (other than a helicopter) under special VFR -
(1) Unless ground visibility is at least 1 statute mile; or
(2) If ground visibility is not reported, unless flight visibility is at least 1 statute mile. For the purposes of this paragraph, the term flight visibility includes the visibility from the cockpit of an aircraft in takeoff position if:
(i) The flight is conducted under this part 91; and
(ii) The airport at which the aircraft is located is a satellite airport that does not have weather reporting capabilities.
(d) The determination of visibility by a pilot in accordance with paragraph (c)(2) of this section is not an official weather report or an official ground visibility report.
[Amdt. 91-235, 58 FR 51968, Oct. 5, 1993, as amended by Amdt. 91-247, 60 FR 66874, Dec. 27, 1995; Amdt. 91-262, 65 FR 16116, Mar. 24, 2000]

91.205(d) basically says you have to be equipped for VFR and have a radio. https://www.ecfr.gov/cgi-bin/text-idx?pitd=20170830&node=se14.1.91_1205&rgn=div8
So clear of clouds and 1 mile visibility - sounds more like scud running to me.

Do you mean that the first link is about takeoff weather minimums, and the second link is about flying within the different airspace classes?

The first is not just takeoff, but flying in those conditions period.  Are you a pilot?

@CrossRoads - Don't most class B airports have NO SVFR as a restriction?

I don't know.  I don't use Class B airports - too much jet traffic, landing fees, high fuel costs.  I went thru the Departures for BOS, which is 25-30 miles away, and the TAKEOFF MINIMUMS, (OBSTACLE) DEPARTURE PROCEDURES, AND DIVERSE VECTOR AREA (RADAR VECTORS) ,and I didn't see No SVFR listed anyplace.

@CrossRoads - Among other places, it's on the sectional chart

Ah, yes it is. I dug too deep it seems.


text:
Are speed limits used for departure turns imposed also on the rest of the procedure?
ICAO's PANS-OPS manual 8168 ("Aircraft Operations") defines turing departures as described below:

2.4 Turning Departures. When a departure route requires a turn of more than 15°, it is called a turning departure.

It also defines maximum speeds used for turning departures.

2.4.4 Turn Speeds. The maximum speeds used for departure turns shall be those of the final missed approach increased by
  10 per cent to account for increased aeroplane mass in departure (see Table II-2-2-1), unless otherwise annotated on the
  procedure.

What I am having trouble understanding is if these speed limits are valid only while the aircraft is performing the turn, and are cancelled during the subsequent straight track of the procedure (only for them to be valid again when another turn takes place).
I am asking because I remember reading somewhere that a speed limit imposed on a fix remains active for the rest of the procedure, unless it is cancelled by another speed restriction.
Thanks for anything that could point me in the right direction.

The point of speed limits on fixes is spacing. The speed limits on turns above are to limit the turn radius. Those are two completely different reasons and there is no reason to think they are connected.

@StephenS Thanks for your input. However, doesn't this imply that the fix where the turn is initiated should have a speed limit less or equal to one of the above maximum speeds? If not, then theoretically the aircraft could perform a turn with a turn radius greater than the maximum allowed.


text:
What is the average climb gradient of an aircraft during the takeoff and early phase of a departure procedure?
On chart plates it is written the minimum climb gradient with which an aircraft should climb for obstacle clearance or after ATC request (for traffic flow management etc.).
However, since this is the minimum required climb gradient, I am curious about what practically happens during real flights. For example, if a chart plate instructs an initial climb with a minimum climb gradient of i.e. 320 feet per nautical mile, will the aircraft try to maintain this minimum climb gradient until the termination altitude? Will it climb with the maximum climb gradient that its performance characteristics allow?
In other words, given an assigned climb gradient from a chart plate (i.e. from DEEZ5: "Standard with minimum climb of 250' per NM to 300), what would the average climb gradient of an aircraft during the initial climb phase be, provided that this value is taken into consideration not only for a lower but also for the calculation of an (hypothetical) upper limit?

The average gradient has very little practical meaning. It varies with aircraft type, engine type, take-off weight, airport altitude, air temperature, engine setting (e.g. derate / assumed temperature), airline procedures etc. etc. The average on one airport can be totally different from the average on another airport, from a summer day to a winter day, from one airline to another. What are you trying achieve?

@DeltaLima I am trying to understand which criteria an aircraft takes into consideration in order to decide its climb rate / climb gradient during the initial climb phase of a departure procedure. In other words, if it always aims to achieve the maximum climb gradient that its performance characteristics allow or if it takes other factors into consideration and, mainly, the minimum climb gradient that is written on a SID plate.

The upper limit is a function of the airplane's performance at rated climb power at the normal speed used, modified somewhat by the wind (the gradient is based on ground speed).  What actually happens on a departure is simply whatever numbers that airplane's performance produces using the standard speed and power setting for that airplane.
In a jet you are generally going somewhere around 200 kt once the flaps are up, then accelerate to 230-250 kt once past 3000 ft up to 10000 ft.
From the table below, assuming no wind, an airliner climbing at 3000 FPM at 200 kt will have a gradient of roughly 1000 ft/nm between the initial acceleration altitude (say 1000 ft) and 3000 ft:

Thank you John for your input. In your example, you mentioned a climb rate of 3000 FPM. I guess that what I am actually asking is if the climb rate is decided based on the mandatory climb gradient which is written on a SID plate e.g. for obstacle clearance purposes. If not, does an aircraft generally try to achieve the maximum climb rate, based on its performance, during the initial climb phase, without "caring" to be close to the SID's minimum climb gradient?

What it means is you just do whatever you normally do without concern for the published gradient, which is a minimum requirement for obstacle clearance, IF you know you can exceed the requirement. Those limits really only apply for marginal cases. If you're in a piston twin flying a departure in a mountainous area that requires a 350 ft/nm gradient, well, no problem cuz my baby can climb well over 526 fpm at 90kt with both engines operating, but what if an engine quits and it can only climb 300 fpm?  If I do the departure and one goes down, I'm in trouble.  Now I have to think hard about it.


text:
Is airport's magnetic variation value used for terminal procedure design?
I am studying how instrument flight procedures are coded to ARINC-424 format, and since I was curious about the method used to determine the outbound/inbound magnetic courses, I checked the section describing the magnetic variation field.

ARINC-424 Version 18, Page 75, section 5.39: […] the government authority has
  established the value as valid for everything associated
  with a given location. For example, if a Magnetic Variation
  of Record is established for an airport location, everything
  referenced to that airport will use the same value. This is of
  interest as it means that Terminal Procedure design is also
  based on that value.

Does this imply that an officially published chart will depict each leg's magnetic azimuth based on the magnetic declination value of the associated airport, rather than the magnetic declination corresponding to the first point of the particular leg of the procedure?
In other words, if I want to find the true azimuth of an aircraft's heading, while it follows a specific terminal procedure, do I substract from the magnetic azimuth depicted on chart the magnetic variation value of the associated airport?

The distances between the farthest point in the procedure and the airfield are small enough that there shouldn't be significant magnetic variation between the two.


text:
Why did the backup landing gear deployment fail on LOT 16?
LOT 16 was a 767 which experienced a hydraulic leak, resulting in the failure of the landing gears and the proceeding belly landing shown below:

Why did the backup system(s) fail? I was under the impression that a gravity/freefall backup is mandatory.

Gravity/freefall backup (or more precisely some backup) is mandatory, but in large aircraft running pulley from cockpit to the wheel wells to release the uplock (the thing that keeps the gear retracted) mechanically would be too complicated or the force required to operate it too large, so the uplock still needs to be released electrically.
Unfortunately the circuit breaker that protects the circuit responsible for releasing the uplock was "popped" (open), so the uplocks didn't release and the gear didn't fall down.
The crew was trying to troubleshoot the issue for 80 minutes and consulted maintenance over radio, but because the checklists for either central hydraulic system (which does the normal gear extension) failure or alternate gear extension did not mention the circuit breaker (C829) neither crew nor maintenance thought of checking it.
It is not possible to find why and when the breaker popped. They tested the alternate extension afterwards and it worked, but the breaker covers several other circuits and there could have been transient problem somewhere or it could have been maintenance error or some other reasons.
You can read excerpt from the report on The Aviation Herald.

Wikipedia says,

Shortly after the evacuation, a team from the Polish State Commission for Aircraft Accident Investigation arrived, and discovered that the C829 circuit breaker, which protects a number of systems including the alternate landing gear extension system was "popped". The C4248 breaker for the alternate landing gear remained closed.
After the plane was lifted off the runway, the C829 circuit breaker was closed, and the landing gear was extended, using the alternate system.


text:
Have there ever been any accidents within holding stacks?
At a busy airport, up to a dozen aircraft waiting to land may fly the same holding pattern at the same time, separated vertically by 1,000ft. — a stack. Some ATC towers have dedicated Stack Controllers.
It's all very routine, but have there ever been any instances of incidents or accidents in stacks?
To the layman, it might seem like stacks could be hazardous — e.g. what if a plane in the middle of a stack lost an engine?

if a plane lost an engine he'd declare emergency and get out the stack, and get priority to land (which they can because well emergency)

Most twin engine commercial aircraft are capable of sustained flight on a single engine.  Its very unlikely a plane in the middle of the stack would suddenly plummet through the stack vertically.  Most likely the emergency aircraft would separate laterally and then get priority clearance to land.

I think he means that engines will hit planes below in the stack!

@HCBPshenanigans That's not what I meant :D

that's what the 1k feet buffer is for, if a plane starts dropping he can get cleared to get out of the stack before he is in the path of the ones below

The article you linked in your question mentions one incident - loss of separation - that's described in more detail here.
The Aviation Herald is a good source of information on incidents and accidents but it has very few reports related to holding. There are several about low-fuel incidents because aircraft had to remain in the hold for a long time (Avianca 52 is a well-known case), but I could only find one that involved a holding pattern more directly and that was also a loss of separation resolved via a TCAS advisory.
Eastern Airlines 401 crashed while in a hold, but it was an ad hoc holding pattern and not in a stack with other aircraft. American Eagle 4184 crashed due to icing accumulation while holding but again there were no other aircraft involved.
I could only find one example of an actual collision in a holding pattern which was over New York in 1960 and in that case only one of the aircraft was holding (inaccurately).

There has been at least one instance of an accident inside a multiply-occupied holding stack.  Fortunately, none of the other aircraft in the stack became involved in the accident sequence.
On 8 December 1963, PA214, a 707-120 en route from San Juan to Philadelphia via Baltimore, entered a holding pattern with five other aircraft already in it due to a squall line hovering over Philadelphia.  Sixteen minutes after entering the stack, the aircraft was struck by lightning, which caused a series of fuel-tank explosions, which caused the aircraft to crash.  At least one of the five other aircraft in the stack saw the burning aircraft falling to the ground, but none of the five were actually struck by the accident aircraft or the detached debris therefrom.
To quote the CAB report:

[...] Flight 214 reported over the New Castle VOR at 2042, 5,000 feet [sic] and control was then transferred to Philadelphia Approach Control which provided the crew with the following:

“... Philadelphia weather, now, seven hundred scattered, measured eight hundred broken, one thousand overcast, six miles (visibility) with rain shower, altimeter two nine four five, the surface wind is two hundred and eighty degrees at twenty (knots) with gusts to thirty (knots).  I’ve got five aircraft, have elected to hold until this ... extreme winds have passed, ... do you wish to be cleared for an approach or would you like to hold until the squall line ... passes Philadelphia, over?”

The crew advised Philadelphia they would hold and were instructed to hold west of New Castle VOR on the 270[°] radial and given an expected approach clearance time of 2110.  The crew requested and received permission to use two minute legs in the holding pattern.  At 2050:45 the crew advised Philadelphia they were ready to start an approach.  They were told to continue to hold and they would be cleared as soon as possible.  The crew acknowledged with “Roger, no hurry, just wanted you to know that ... we’ll accept a clearance.”  Approximately eight minutes later, at 2058:56 the following transmission was heard on the Philadelphia Approach Control frequency 124.6 “MAYDAY MAYDAY MAYDAY ... Clipper 214 out of control.  Here we go.”  Seconds later another transmission on the same frequency was heard “Clipper 214 is going down in flames.”  This latter transmission was made by the first officer of National Airlines Flight 16 (NAL 16). ...  NAL 16 was in the same holding pattern as Flight 214 but 1,000 feet higher, and the first officer had seen the Pan American flight descending on fire. [Page 3; emphasis mine.]


text:
Can you legally obtain Boeing maintenance documents?
With the correct keywords on Google, it is possible to find Boeing MPDs (Maintenance Planning Documents), as well as Task Cards, and other technical documents.
I'm wondering, could a software vendor with the need to access these documents legally obtain them?

I assume that "anyone" can legally obtain them by buying the corresponding plane. Could you be more precise?

@DavidRicherby Buying a 737, a 747, and a 777 - for example - is clearly unfeasible for a software vendor with just the need to access technical documents.

Sure, it's obviously infeasible to buy the plane to get the manual but you don't give any indication of what would be feasible.

@DavidRicherby Some common sense is needed...

@DannyBeckett and thinking it would be illegal to own those manuals is common sense?

@jwenting They generally have copyright notices on them, at a minimum.

@DannyBeckett so do the novels I buy regularly from Amazon. Those mean you can't make copies and sell them, not that you can't own the thing...

Everything is available for a price, and you don't need Google's help: You can get the Boeing Maintenance Performance Toolbox right from Boeing if you contact them.

You can also Google around and get PDFs of Boeing maintenance documents but these would be from unofficial/unauthorized sources (at least one Boeing maintenance manual made it onto Wikileaks).
As Boeing likely holds a copyright on their maintenance documents obtaining them from an unofficial/unauthorized source would be an infringement on that copyright.
More importantly you can't guarantee documents from an unofficial source are correct, current, and unmodified. If you intend to use the document for any aviation purposes (like performing maintenance on an aircraft) you would want to be absolutely certain they are the latest approved revision from Boeing.

I'd love to know roughly how much this costs...

@DannyBeckett I would assume "A Lot." -- The service manual pack for Piper Cherokees runs about $600, and the docs for a Cessna 172 will set you back over $1000.

The majority of maintenance manuals are customised for a specific fleet, i.e. specific to a particular airline.
That way Boeing only provides information that is applicable to those aircraft and even with a given manual you need to be able to filter on applicability to ensure that specific maintenance information is applicable to a specific aircraft.
Having worked with airlines using Boeing maintenance data, the position pre-B787 was that the maintenance manuals were owned by the airlines but that Boeing were responsible for the original content. Boeing seem to be trying to change that for the B787, possibly in an attempt to get all B787 customers to use their Maintenance Performance Toolbox authoring/consultation tool instead of being able to use third party solutions or even Airbus's Maintenance documentation solution
So back to the original question, the answer is Yes , if an airline gives it to you, or No if you go knocking on Boeing's door.


text:
Is it legal for a non-IFR rated pilot to fly a SID on a VFR flight?
Just wondering in case the pilot is unfamiliar with the airport / terrain or is flying at night and wants obstacle / terrain separation.

It depends what you mean by 'flying a SID'. If it is just following the procedure without violation of VFR (i. e. you are above VFR weather minimums for a given airspace, you maintain your own separation from traffic, don't violate ATC clearances, don't enter controlled airspace without a clearance etc.) - yes, it's perfectly legal. You can fly any route you want, your choice (again, I assume that you do not violate ATC clearances).
If you mean by 'flying a SID' obtaining an ATC clearance to fly a SID - if it is an IFR clearance, than no, it is illegal. You cannot request IFR clearance if you or your aircraft is not IFR rated, except the cases when you are training for an IFR (and have a CFII next to you).

More specifically, I wanted to know if I could fly both the lateral and the vertical portion of the SID ("climb via SID") while on a VFR flight and continuing to abide by VFR rules (cloud and visibility requirements in addition to the requirements you mentioned).

Yes, you can fly any path you want if it complies with ATC instructions, VFR restrictions, airspace restrictions etc., but you cannot request ATC clearance for flying SID until you are instrument rated or training for your IR and have a CFII aboard, and your aircraft is also IFR-approved.

You'd still have to follow VFR procedures, but nobody is going to stop you from flying the routing of an IFR procedure up to the point where there are published VFR procedures (within the restrictions imposed on VFR flights of course. E.g. where I train VFR is only allowed up to 1500ft, and the immediate area around the airport has specified VFR approach routes and pattern turn locations which don't match the IFR charts for the field in any way).
Of course if you're training for an IFR rating different rules apply.


text:
How to fly IFR to an airport without any instrument approaches?
Would I just file an IFR flight plan to the airport in question, and expect to be cleared for the visual approach if there are no instrument approaches at the destination airport?

Maybe by filing a Y flightplan.

You can end your IFR flight at any point and switch to VFR.

In the US, when you file, you must include an alternate that satisfies the requirements of §91.169  e.g Precision 600 and 2, non-precision 800 and 2, no approach—visibility minima are those allowing descent from the MEA, approach, and landing under basic VFR.

Yes.  If the airport in question has no operating control tower you need to cancel your ifr flight plan after landing if you choose to not do it in flight.  Once you land you can cancel with air traffic contol, if you are able to establish radio communication, or cancel with a flight service station.  It is usually going to be easiest to cancel with the local approach controller in flight, if there is no operating tower.  Remember all ifr traffic, landing and departing, will not be able to receive their approach clearance or ifr release until you cancel.


text:
Are all IFR flights assigned squawk codes in US controlled airspace?
In the US, all IFR aircraft need a clearance to be in class A, B, C, D, and E airspace. Does this mean that all aircraft with clearances need to be assigned squawk codes by ATC when they enter?

No.
/X (VOR only), /D (VOR/DME only), /Y (LORAN only), and /M (TACAN only) aircraft do not have transponders. Primary-only aircraft can be assigned flight plans, and fly IFR. It's not common, though, because aircraft that will fly within 30 miles of class B airspace, or aircraft that will operate above 10,000 feet MSL are required to be equipped with transponders and Mode C altitude reporting, similar to the ADS-B requirements. (14 CFR § 91.215)

One downside of not having a transponder is that ATC will most likely require that you report at all the mandatory reporting points—and most pilots only vaguely remember how from studying for their IFR rating. I did have the transponder fail when doing practice approaches for my IFR. ATC had us do some turns, identified us on their scope, and then everything was like normal.

What are D, M, Y and X aircraft?

@pcfreakxx When you file a flight plan, you need to indicate what type of navigation equipment and transponder you have on board by appending a suffix to the aircraft type, for instance, "C172/X" would be a Cessna 172 Skyhawk with no navigation equipment at all. Here is a list of all the codes.

@HiddenWindshield I understand, you are talking about equipment codes. You may want to edit your answer for clarity

@pcfreakxx I wasn't the one that posted the answer.

@HiddenWindshield I‘m referring to atc_CD :)

Edited. @HiddenWinshield, /X is not "no navigation equipment at all", really, even though we've been taught that for as long as I can remember. /X is VOR equipped, but no DME or transponder. To fly IFR, minimum navigational equipment is VOR and ADF.

@JScarry, in 2011 (not long before I retired) I had a non-radar aircraft come on my frquency, with this - "Denver Center, N12345 (not his real name) one-five miles west of Crazy Woman V-O-R, at one-tree-thousand, estimating Crazy Woman at one-eight-fower-tree, Gillette next."
I was impressed.

/X doesn't necessarily mean "nothing at all". The bare minimums for IFR flight are an ADF and a VOR receiver.  Very few aircraft are set up this way, and they're usually survivors or restorations that can fly IFR, but rarely do so.

Yes.
Every aircraft needs to be assigne a discrete code to allow radar systems to correlate flight plan data with the radar target and to enable identification using SSR.
The only exemption are aircraft which use Mode S, those can be assigned squawk 1000 and this leads to a correlation of flight plan data using the callsign transmited by the transponder instead of the squawk itself.


text:
What is the "one-in one-out" FAA rule for IFR traffic landing or departing at an airport without a tower or at an airport with a closed tower?
Does the rule say that only one IFR aircraft may land or depart at an airport at one time? I'm having trouble finding the official rule in an FAA document.
What about other VFR traffic that may be flying in the traffic pattern doing practice touch-n-goes? Would that activity be affected by the rule?

Depends on city

At an nontowered field - when you are cleared for an approach you are the sole owner of that airport until you report landing or go missed. If there is VFR traffic in the pattern, ATC will advise you of the traffic, you are responsible for avoidance.  Of course there will be little chance of pattern traffic if the ceiling is at 500 feet.

Thanks Mike - does that mean that only one aircraft can be cleared for an approach for ANY runway at an airport, OR that one aircraft can have a take-off clearance for an airport on ANY runway? ATC can't issue both a take-off and a landing clearance at the same time, for the same nontowered airport, correct?

ATC will never issue a landing clearance nor a takeoff clearance for a non-towered airport. They will issue an approach clearance or a departure release, respectively. That is a very important distinction.

Several sections of the AIM give guidance on this. It is explicitly stated that only one aircraft at a time is given a departure clearance. It is implied that there would be no approach clearances until the departing aircraft’s position is known by ATC.

4−4−1. Clearance
a. A clearance issued by ATC is predicated on known
  traffic and known physical airport conditions. An ATC clearance means
  an authorization by ATC, for the purpose of preventing collision
  between known aircraft, for an aircraft to proceed under specified
  conditions within controlled airspace.
5−2−6. Departure Restrictions, Clearance Void Times, Hold for Release,
  and Release Times a. ATC may assign departure restrictions, clearance
  void times, hold for release, and release times, when necessary, to
  separate departures from other traffic or to restrict or regulate the
  departure flow.
  1. Clearance Void Times. A pilot may receive a clearance, when operating from an airport without a control tower, which contains a
  provision for the clearance to be void if not airborne by a specific
  time. A pilot who does not depart prior to the clearance void time
  must advise ATC as soon as possible of their intentions. ATC will
  normally advise the pilot of the time allotted to notify ATC that the
  aircraft did not depart prior to the clearance void time. This time
  cannot exceed 30 minutes. Failure of an aircraft to contact ATC within
  30 minutes after the clearance void time will result in the aircraft
  being considered overdue and search and rescue procedures initiated.
NOTE−
  1. Other IFR traffic for the airport where the clearance is issued is suspended until the aircraft has contacted ATC or until 30 minutes
  after the clearance void time or 30 minutes after the clearance
  release time if no clearance void time is issued.
5−5−4. Instrument Approach
b. Controller.
  1. Issues an approach clearance based on known traffic.
5−5−4. Instrument Approach a. Pilot.
  1. Be aware that the controller issues clearance for approach based only on known traffic.

This makes sense in IMC since two IFR flights may not be able to see each other. Buy what about in VMC, when there may be VFR flights in the area (possibly below radar) and IFR flights are getting visual approaches?

@StephenSprunk All aircraft in VFR conditions are responsible for seeing and avoiding other traffic. That’s why airports with instrument approaches have the approach contained in Class E airspace so that you have a chance of seeing VFR traffic when you break out of the clouds.

Logically, the point of the one-in/one-out rule is to cover cases when IFR aircraft can't see each other; if they can, though, then does the rule not apply?  Or is this one of those (many) cases where logic and rules diverge?

@StephenSprunk I don’t think logic and rules diverge, there are just different rules for IFR separation and VFR separation. Controllers are required to keep IFR traffic away from other IFR traffic. They may not even know about VFR traffic so they can’t provide separation. e.g., The ceiling can be 1500' with 3 miles visibility and in a Class E airport you can be doing touch-and-goes in the pattern. ATC may not have coverage to the ground at that airport, but if everyone is playing by the rules, the plane in the pattern is at least 500' below the clouds giving you time to see and avoid them.

The rule is found in FAA Order JO 7110.65 Air Traffic Control, which is the internal job order laying out the rules that air traffic control specialists must adhere to when providing services.
To begin with there is this subparagraph from Section 4–8, Approach Clearances, which applies equally at towered and non-towered airports:

4–8–1 Approach Clearance
f. Except when applying radar procedures, timed or visual approaches, clear an aircraft for an approach to an airport when the preceding aircraft has landed or canceled IFR flight plan.

So even at a towered airport, the default rule is that only one aircraft may be cleared for an approach at one time. The next aircraft cannot be cleared for an approach until the tower controller reports to the approach controller that Aircraft #1 has landed. Of course "radar procedures" (Section 5–9) are used at almost every towered airport in the NAS, and where those are not available there are "timed approach" procedures specified in Section 6–7.
But at non-towered airports the "one-in one-out" rule cannot be bent, not even using the "except when applying radar/non-radar/visual rules" exception from 4–8–1f. This is because of another set of rules (which also apply at both kinds of airports) concerning overdue or unreported aircraft:

10–4–1 Traffic Restrictions
IFR traffic which could be affected by an overdue or unreported aircraft must be restricted or suspended unless radar separation is used. The facility responsible must restrict or suspend IFR traffic for a period of 30 minutes following the applicable time listed in subparagraphs a through e.

10–4–3 Traffic Resumption
After the 30-minute traffic suspension period has expired, resume normal air traffic control if the operators or pilots of other aircraft concur. This concurrence must be maintained for a period of 30 minutes after the suspension period has expired.

(Note that unlike 4–8–1f, which only deals with successive arrivals, these rules concern all IFR traffic; each paragraph specifically references paragraph 4–3–4 concerning departure releases and void times.)
At a towered airport, the tower controller can observe the arriving or departing aircraft being in a correct position and operations can continue unrestricted. But at a non-towered airport, there is no-one to issue a landing clearance and observe the aircraft safely touch down; there is no-one to issue (or withhold) a takeoff clearance. Therefore every IFR aircraft which has been given authorization to arrive at or depart the airport must be considered "unreported" until proven otherwise. This is the true basis for the "one-in one-out" rule.

I mean that the tower controller has the aircraft in sight and can observe that they have not crashed, nor are they in a position to affect with other IFR traffic. In other words at a towered airport the FAA, in the person of the tower controller, has a direct view of the airport and the local traffic situation; at a non-towered airport the overlying IFR controller does not have this direct view, and has to assume the worst-case scenario until informed otherwise.


text:
What does the letter G mean in a runway identifier?
Boulder Municipal Airport (KBDU) has runways 8 / 26 and also runways 8G / 26G. What is the significance of the letter G following the runway numbers?

Possible duplicate of How are runways numbered?

@GdD I don't agree on the duplicate, there is no mention of any "G" in that linked question/answers.

It is in there @Federico, look at Airhead's answer.

@GdD ok, missed it. I'm evaluating editing to make it a bit more prominent, now it is really buried and hidden.

@GdD one paragraph in the 4th answer hardly makes it a dupe. SE is supposed to make it easy to find the right answer, not a puzzle.

I marked it as a duplicate because it had comprehensive information on runway numbering, and included an answer to the OP's question, but I'm happy to go with the community on this one.

For completeness' sake: This looks like a US-ism. As counterexamples, my Swedish airport directory lists for example Karlskoga ESKK with a 1499 x 30 m asphalt runway and a 900 x 40 m grass runway, both designated 03/21; or Gävle ESSK with a 2000 x 45 m asphalt runway and a 730 x 30 m grass runway, both 18/36. Not even any R/L designators with either of these; just two different, parallell runways with different surface characteristics and the same designations.

@MichaelKjörling That sounds crazy. I could understand using L/R, but using the same designator for two different runways at the same airport sounds crazy. Seems like it could lead to errors, especially in IMC and/or at night where it's not easy to see the difference between surface types from a distance.

@reirab I could be wrong, but I very strongly suspect that few grass strips (at least around here) are used in practice in IMC or at night, so the issue may be mostly academic. My local airport has three parallell landing/takeoff areas; one asphalt runway with one grass strip on each side of it, all technically known as 04/22. The distinction is made by specifying e.g. "runway 22" or "eastern strip 04" or "western strip 22". Maybe "22C" or "04R" or "22L" would be clearer for outsiders, but given that the airport doesn't even have the equipment to support instrument approaches...

@MichaelKjörling Oh, I agree that someone wouldn't likely intentionally be using a grass strip in IMC. My concern was more for unintentionally lining up on the grass when the asphalt was intended if they had the same runway designation (e.g. accidentally selecting the wrong one in a GPS system or similar.)

@reirab -- the grass strips wouldn't be programmed into GPS or FMS databases to begin with, though (as they aren't instrument runways at all)

For runways with other than hard surfaces it is common to list the runways with a letter G for grass or in the case of a seaplane base, with W for water.
An example of this is runway 8G at Boulder (KBDU) and runway 35W at David Wayne Hooks (KDWH).
You can see this information in the Chart Supplement pages and diagrams for each airport.

Such as 21W in Manhattan.  Once, USAir served it.

This answer is not technically correct, the surface type of the runway is not indicated in the designation. It is a coincidence that "W" type runways are water, the suffix indicates type of runway not surface type. S = STOL runway, G = glider runway, W = water sealane or waterway, and U = ultralight runway. I would have commented sooner  but I needed to build up some reputation, so I made my own answer.

For a moment, when reading this, I thought you cited a suffix that meant that the runway had boulders.  Took me a moment to realize my mistake, but it did lead me to think up some really amusing airports before I realized it!

@Harper I suspect they quickly realized that it would be difficult to turn a profit on that particular one. Also, I imagine the landing fees were kind of stiff in the end.

The "G" suffix refers to a Glider runway.
The following verbiage is used in Advisory Circular: AC No: 150/5200-35  Page 15 RUNWAY IDENT
"The following suffixes can be used in conjunction with runway identification numbers even if the runway is not painted accordingly: S = STOL runway, G = glider runway, W = water sealane or waterway, and U = ultralight runway"
Interestingly runway 8G / 26G is actually a dual surface runway with part of it being paved and the remainder grass.
The following is the description of that runway: Found here AirNav: KBDU

Runway 8G/26G Dimensions:     4100 x 20 ft. / 1250 x 6 m
  Surface:   asphalt/turf, in fair condition
  RWY 08G/26G 3,000X20 FT CHIP AND SEAL ON WEST END; EAST 1,100 FT IS GRASS.
  Gradient:  W
  Operational restrictions:  RWY 08G/26G LNDG AREA UNDEFINED.
  RUNWAY 8G      RUNWAY 26G
  Latitude:  40-02.378333N       40-02.378333N
  Longitude:     105-14.001500W      105-13.122833W
  Elevation:     5287.0 ft.      5274.0 ft.
  Gradient:          0.3% UP
  Traffic pattern:   left        right
  Obstructions:  60 ft. trees, 200 ft. from runway, 126 ft. left of centerline,  3:1 slope to clear      none

Welcome to Aviation.SE! Nice answer. We hope to continue seeing you around providing more of them!


text:
What is the difference between the red and black box identifiers on UK airport charts?
On UK airport charts like the one below for Manchester (EGCC), what is the difference between an identifier with white lettering on a red square and one with yellow lettering on a black square with a yellow outline?

(eurocontrol.int)


text:
Does clearance delivery assign VOR or GPS-A approaches?
I've listened to clearance delivery feeds on LiveATC. Is it possible for clearance delivery to assign VOR or GPS-A approaches instead of standard terminal arrivals (STARs)? Or would they just give you routing to a VOR or another waypoint near the arrival airport, and say "direct" afterwards to indicate that the TRACON for the arrival airport will assign the specific approach?

No.  Normally you don't know which approach you are going to be using at your destination because you can't be sure which runway will be in use.  The STARs at larger airports are to provide standard routings for all/most the of airport's approaches/runways and each STAR is based on a different arrival direction, so the STAR in your clearance will usually be based on where you're coming from and you usually include one in your filed flight plan, that you've picked because its initial waypoint links in nicely to your route.  The actual routing within the STAR you will be using, you find out when you listen in on the destination ATIS to find out the approach in use as you plan your arrival.
Once you get there, you may only fly one or two legs of the STAR before ATC gives you a heading and now you are on radar vectors to get lined up for the approach and you just do what you're told and the rest of the STAR routing goes out the window.
The key thing in IFR is accommodation of communication failures. Comm failure procedures are intended to allow you to complete your flight by following your most current clearance and observing SOPs that allow ATC to predict where you're going to go and when, with reasonable confidence, so that they can work traffic around you.  If you've declared a comm failure (7600) enroute, you would fly the STAR routing and altitudes as published and use as much common sense as possible.
For example, a lot of STAR routes kind of drop you off at a Downwind Termination Waypoint for a particular runway and indicate radar vectors beyond that.  In a comm failure situation you are expected to fly the published route to the DTW and from there you are expected to find your own way to the approach by the most logical, predictable routing that causes minimum disruption.
If the destination has no STAR, your clearance will be to the last waypoint in your route that gets you close to the airport, and that will be your clearance limit and from there you would get radar vectors to line up with whatever approach is in use.  In a case of a comm failure, you are supposed to proceed to your clearance limit, hold there if you're early, then proceed to fly the appropriate approach at the time specified in your flight plan by the most logical routing, so that ATC can predict where you're going and when.

So if the destination airport has no STARs, your clearance limit won't be the airport. And when you get to your clearance limit, they will give you radar vectors and amend your clearance limit. Why not issue you a clearance limit all the way to the arrival airport right off the bat, instead of giving it to you later?

No because you aren't going to just fly to the middle of the airport.  You're going to fly to a fix that places you in a position to proceed to an instrument approach to that airport.  When you get the initial clearance, it's gotta stop somewhere short of the airport itself because you don't know which way you're going to proceed until you are close enough to be able to plan for a particular approach to a particular runway, and when you get there, further clearance is necessary. That's the clearance limit - your legal authorization to travel to point X at the time you depart.


text:
If I am transiting a class delta airspace on a VFR flight and am coming up to a class bravo boundary without a bravo clearance, what will happen?
Kind of an edge case, but will the class delta controller ask me to "say intentions"? Or maybe let a bravo controller know I could be entering without permission?

Because you're in Delta space, you are necessarily talking with a Delta controller.They will likely do one of two things:
Either:

give you a vector to keep you clear of Bravo:
N45WTF:  Turn left 40 degrees
coordinate with the Bravo controller, and the Delta controller may give you clearance:
N45WTF:  Climb and maintain 3,500.  Squawk 3462, Cleared into the Bravo, contact Center on 118.9

Thanks. If the delta controller doesn't coordinate your bravo clearance, how does an aircraft legally remain in the delta when switching frequencies to contact the bravo for clearance (to be in class delta airspace, pilots need to establish AND maintain two-way communications)? Maneuvering the aircraft to leave the class delta airspace to contact the bravo controller sounds like too much of a hassle to me, but then again rules are rules.

"Tower:  N45WTF requests frequency change to Center"  (tower will respond with:  "Frequency Change Approved, switch to 118.9").  If you request it, and they approve it, you're allowed to switch frequency.

More than likely, you would be talking to Approach before entering the Delta if they are that close. You will talk to Delta, they will arrange the clearance into Bravo before handing you off, or hand you off to Approach for Bravo and tell you to remain clear.


text:
What are a few main differences between ATC interactions for rotorcraft vs fixed-wing?
The title says it all. In particular, I'm wondering what the differences are in reference to:

Takeoffs and landing - can rotorcraft be cleared to take-off / land directly from / to the helipad?
When rotorcraft want to head VFR to a location, are they treated like normal fixed wing aircraft (e.g. given an altitude and / or heading restriction)?
In fixed-wing IFR, aircraft are sometimes put into a holding pattern. If a rotorcraft is asked to hold, is it legal for them to hover at whatever location they want?
I think that helicopters are sometimes asked to surface taxi / hover taxi / air taxi at airports to keep everything in order.

Feel free to add any other differences between ATC interactions between rotorcraft and fixed-wing I might have missed.

A helicopter can be cleared to depart from any position on an airfield, however there are a lot of considerations that would go into that.
There is no regulation in the US, and I'm almost certain there isn't an ICAO SARP that states a rotary winged aircraft must depart from a designated position on the airfield. Based on local conditions, volume & types of air traffic, and how familiar controllers are with handling a helicopter, there may be some localized procedures, but nothing regulatory.
As far as hovering, there are no prohibitions aside from local airfield procedures. If given a hold during an IFR flight, the rotary wing pilot will fly it in the same fashion as a fixed wing aircraft. To hover in a specific spot for an extended period of time at altitude of not practical due to lack of visual cues needed to hover. Some have a system of Auto-Hover, but that's generally used for extended hovering close to the ground.
Even if it were desired, in many cases it wouldn't be possible. Hovering requires a significantly higher amount of power when done so outside-of-ground-effect (OGE), which is a key metric you need to understand prior to every flight based on that day's conditions and how heavy you are. The extra power required burdens more gas, and in many cases requires a good percentage of the pilots available attention.
If burning the extra gas isn't enough, most civilian rotary-wing aircraft are not equipped with the power needed. Even those military helicopters with two engines (think UH-60 Blackhawk) are limited at higher attitudes.
I think that was all the questions.

So what you are saying is that many civilian helicopters (say a Robinson R22) do not have enough engine power to hover outside of ground effect? I thought that the ability of hovering "anywhere" was the whole definition of a helicopter.

Could you please carefully re-read your post and fix all the auto-correct-induced typos?

MadMarky, helicopters greatest capability is vertical takeoff and landing. Hovering is second to that, especially as altitude.


text:
Has Jeppesen stopped providing ARINC424 data?
As far as I am aware of, Jeppesen appears to be the main distributor of navigational data (Airports, SIDs, STARs, IAPs etc.) coded in ARINC424 format. However, I can not find on its site any product based on the aforementioned digital format. Moreover, all related links are currently broken (e.g. ARINC 424 NavData Services, previously active based on google cache).
Could someone shed some light on this?

Try searching by "ARINC 424" instead of just one word.

@JuanJimenez thanks for your input. I have already tried different approaches regarding the possible keywords, but they yielded no results. In case you are aware of a valid relevant link, could you point me in the right direction?

Jeppesen is indeed still the main provider of ARINC 424 navigation data to OEM customers (mainly avionic suppliers). You could look at the following links on Jeppesen Web:

Jeppesen NavData® and Geospatial Data
Jeppesen Avionics Solutions

Other suppliers include Lufthansa and Navblue


text:
Based on PANS-OPS, what is the earliest turning point that can be used in a missed approach?
I am studying ICAO's PANS-OPS 8168 manual, and I have noticed conflicting guidelines regarding the permitted earliest turning point during a missed approach.
In section 6.2.1.1, it clearly states:

6.2 Climb Gradient and MOC. 6.2.1 Initial Phase. 6.2.11. The initial phase begins at the earliest missed approach point (MAPt) and ends at the start of climb point
  (SOC). The manoeuvre during this phase requires the concentrated attention of the pilot, especially when establishing
  the climb and the changes in configuration, and it is assumed that guidance equipment is not utilized during these
  manoeuvres. No turns may be specified during this phase.

A few pages later, it also states:

6.4.7 Turn Specified at the MAPt. Where the turn is specified at the MAPt, which means that the pilot is supposed to establish the aeroplane on a climbing
  path and then to turn, the OCA/H will be taken as the turn altitude/height and the turn initiation area will extend from
  the earliest MAPt to the SOC (see Figures I-4-6-10 and I-4-6-20).

However, in Figure I-4-6-10, a turn less than 15 degrees is depicted to begin immediately at MAPt, rather than at SOC. On the other hand, turns more than 15 degrees seem to begin at SOC as it is intended (Figure I-4-6-20).

Additionally, in section 6.4.5 (Turn initiated at a designated altitude height), it states:

6.4.5.2.1.1 The point where the designated altitude/height is reached is not fixed. It depends on the climb
  performance of the aircraft and the point from which the missed approach is initiated. The aircraft may reach the
  designated turn altitude/height:
a) as early as the earliest MAPt when the procedure prohibits turning before the MAPt or as early as the earliest
  FAF when no restrictions are provided
b) after a climb using the minimum required gradient from the SOC to the point where it reaches the specified
  altitude height. This point is called the Turn Point (TP).

In Figure I-4-6-11, one of the potential turning points is depicted near the MAPt, well before SOC.

All things considered, I can not figure out the earliest point where turning is allowed to be initiated, during a missed approach.
Could someone help me clarify this?

I would simplify it and not turn before the MAP point.

I can not figure out the earliest point where turning is allowed to be initiated, during a missed approach.

It is up to the designer. Nothing in PANS-OPS prohibits turns before MAPt but in that case an additional area extending from the earliest FAF needs to be protected as shown in figure I-4-6-14 PANS-OPS Vol. II.
Because early turns are not always feasible, advisable or convenient, we often disallow turns before MAPt, in which case a note to the effect will be added on the profile view of the approach chart. Any turns >15° will then be executed in the final missed approach area (Vol. II I-4-6-10 6.4.6.3.1).